In particular, the invention is intended for use in a method of recording in which a thin film of thermoplastic material is carried on a conductive support, a track of electric charge is laid down on the film by means of a scanning electron beam modulated in accord with an input signal, the film is heated either before or after the track is deposited and is subsequently cooled or allowed to cool to preserve variations in a contour of a groove formed along the line of the track. The track is preferably a spiral track in order to provide conveniently a substantial playing time of a recording. However the present invention is applicable generally where an approximately circular area is scanned and thereby carries a recording.
The mechanism by which signals are recorded by this method is as follows: if the material is softened the electrostatic attraction between the deposited charge and the conductive support produces physical contours corresponding to the input signal; subsequent cooling of the thermoplastic material makes possible the preservation of the contour recording despite the eventual dissipation of the deposited charge.
The present invention is particularly concerned with the heating of the film. It is desirable that all the conditions which affect the deformation of the film are kept constant over the recorded area, and therefore that the film should reach a uniform temperature so that it responds in a uniform manner over the recorded area to the electrostatic pressure exerted by the deposited charge.
The term "thermoplastics" is not used herein in a strict sense to connote any specific group of chemical compounds; a variety of substances, including waxes, would be suitable. One thermoplastics material that would be particularly suitable is polystyrene which can be sprayed, painted or spun onto the conductive disc and, if necessary, partially scraped off to leave a film of the desired thickness.